Module 2. Classification of dairy associated microorganisms
MORPHOLOGY AND CLASSIFICATION OF DAIRY BACTERIA
Micro-organisms found in milk can be studied as groups possessing one or more major characteristics in common. These may be divided on the basis of their action on milk constituents, for example acid-former, proteolytic, saccharolytic etc. Another approach of classification is based on the morphology of these micro-organisms. These types of classification of dairy industrial microbes have been discussed in the following sections in this lesson.
4.2 Classification Based on Shape, Size and Arrangement of Cells
The bacteria have been classified as cocci, bacilli, spirilla based on the shape of the cells. However, some bacterial cells appear in different shapes or may be lacking uniform shape and are known as pleomorphic (e.g. Arthrobacter, Bifidobacterium) (Fig. 4.1). Further based on the arrangement of cells bacteria are also classified into different groups. The arrangement of cells linked side by side similar to that like matchsticks is known as palisade arrangement.
Bacterial cells with spherical or ellipsoidal shape are called as cocci and may have the following type of arrangement of cells:
a) Diplococci: cells divide in one plane and remain attached in pairs (e.g. Neisseria)
b) Streptococci: cells divide in one plane and remain attached after some divisions, in form of chains (e.g. Streptococcus)
c) Tetrads: cells divide in two planes and form 4 cells (e.g. Pediococci)
d) Sarcinae: cells divide in 3 planes and form a cuboidal arrangement
e) Staphylococci: cells divide in 3 planes in an irregular pattern producing bunches of cocci.
Bacteria with cylindrical or rod like cells are called as bacilli and these may have the following type of arrangement of cells.
a) Diplobacilli: cells divide in one plane and remain attached in pairs.
b) Streptobacilli: cells divide in one plane and remain attached after some divisions, in form of chains.
Ends may be rounded as in Lactobacillus bulgaricus or squamosed as in Bacillus anthracis.
Bacteria with spiral or helical shaped cells are called as spirilla and may have the following characteristic shape.
Vibrio: rod shaped cells having single curves
Spirochetes: rod shaped cells having many curves
Few curves: true spirilla
4.3 Classification Based on Temperature
Microorganisms capable of growing between 20 and 40°C with the optimum growth temperature of 37°C are termed as ‘Mesophiles’. All pathogenic organisms are mesophillic in nature (e.g. S. aureus, E. coli).
Microorganisms that are capable of growing at or below 7°C (refrigerated) but the optimum growth temperature are between 15 and 20°C are termed as psychrotrophs. These are the significant spoilage organisms of refrigerated milk and milk products (e.g. Pseudomonas spp., Alkaligenes spp.).
Organisms capable of growing over 50°C with optimum growth temperature of 55°C are termed as thermophiles. They are the important organisms causing outbreaks in heat processed milk and milk products. Some are capable of growing between 40-85°C. Organisms produce enzymes at rapid rate, so that enzymes are replaced quickly (e.g. Bacillus stearothermophilus, Streptococcus thermophilus).
Organisms capable of withstanding pasteurizing temperatures of 63°C/30 min. with optimum growth temperature of 35-37°C are termed as thermodurics. They form important flora of pasteurized or heat processed foods (e.g. Micrococcus varians).
4.4 Classification Based on Oxygen Requirement
Organisms capable of growing in the presence of oxygen are termed as aerobic organisms. They can grow in a standard air atmosphere of 21% oxygen. They are more efficient in utilization of available nutrients (e.g. Bacillus spp.).
Organisms which cannot grow in the presence of oxygen but can grow in the presence of CO2 are termed as anaerobic organisms. They do not use O2 for energy yielding reactions. They are however poisoned by O2. Some tolerate low concentrations of O2. They produce catalase and peroxidase enzymes.
High tolerance – Clostridium perfrigens
Moderate tolerance – Clostridium tetani
Organisms which can grow either in the presence or absence of oxygen are termed as facultative organisms (e.g. E. coli, Lactococcus lactis ssp. lactis).
Organisms which grow best at 1-15% of O2 levels are known as microaerophilic. They can use O2 for energy yielding reactions but cannot withstand more than 21% of O2 (e.g. Campylobacter jejuni).
4.5 Physiological Grouping
4.5.1 Acid producers
The organisms capable of fermenting lactose to form lactic acid are known as acid producers. Lactic acid coagulates milk by producing precipitation of Casein at 4.6 pH.
Homofermenters: Lactococcus, few Lactobacilli
Heterofermenters: Few Lactobacilli, Lueconostoc spp.,
LAB are classified as heterotrophic chemoorganotrophs i.e. these require the preformed carbon and energy sources (Fig. 4.2). LAB lack cytochrome and cannot use respiratory pathway for energy generation. LAB generally lack metabolic diversity and mainly relies on two types of fermentation for the metabolism of sugars that form the basis of their taxonomic characterization. Based on Embden Meyerhoff Pathway and ‘Phosphoketolase’ pathways LAB are classified as follows:
· Homofermentative ‘LAB’: These Homofermentative ‘LAB’ group use ‘Embden Meyerhoff Pathway’ for hexose sugars utilization and produce 2 pyruvate and 2 ATP molecules. Pyruvate is further converted to lactic acid by the enzyme ‘lactate dehydrogenase’ and is the only end product. However, under certain conditions the homofermenters can also undergo heterolactic fermentation. For example: during pentose sugar fermentation by ‘LAB’.
· Heterofermentative ‘LAB’: This group of bacteria catabolise hexose sugars by ‘Phosphoketolase Pathway’ and produce an equal amount of lactic Acid, carbon dioxide, acetate and ethanol as the major end products (Fig. 4.3)
· During this process only one ATP molecule is produced.
4.5.2 Gas producers
Gas producers are the organisms capable of producing CO2 or \ and H2 from lactose fermentations (e.g. E. coli, yeasts, and Clostridium spp.).
Proteolytic organisms degrade milk proteins into soluble components by enzymes known as proteinases or proteases (e.g. Bacillus spp, Pseudomonas spp.).
Lipolytic organisms are capable of attacking milk fat by enzymes such as lipases liberating glycerides and fatty acids (e.g. Pseudomonas spp, Achromobacter lipolyticum; Moulds: Geotrichum candidum, Penicillium roqueforti).
4.5.5 Sweet curdling
Organisms capable of causing curdling of milk by rennin like enzyme before the development of sufficient acidity are known as sweet curdling organisms (e.g. B. subtilis, B. Cereus and Enterococcus liquefaciens).
Ropiness organism produce change in the viscosity of milk or forming threads when the milk is poured from one container to other, due to production of gums, mucins etc. (e.g. Alcaligenes viscosus).
4.5.7 Flavour producing
Many organisms are capable of producing different flavours in milk during their growth. Some of the flavours and associated organisms are listed below:
Fruity – Pseudomonas fragi
Malty – Lactococcus lactis subsp. lactis biovar maltigenes
Fishy – Proteus icthyosmius
Unclean – E. coli
4.5.8 Colour fermentations
Many organisms are capable of producing different colours in milk during their growth by producing chromogenic compounds. Some of the colours and associated organisms are listed below:
Yellow: Pseudomonas synxantha
Blue: Pseudomonas cyanogenes
Green: Penicillium roqueforti
Black: Pseudomonas nigrifaciens